1. Field of the Invention
The invention is directed to a method for operating a converter circuit with voltage boosting.
2. Description of the Related Art
Electrical three-phase generators serve for generating electrical energy that is generally fed into a supply voltage system. In the case of specific generators, e.g., in wind power installations dependent on the instantaneous wind conditions, their output speed, power or voltage can fluctuate, and is neither predictable nor controllable. Since the voltage supplied by the generator can thus lie below the voltage of the system to be fed, a boost converter (boost) circuit is often connected between generator and system. Particularly for generators having a center connection, so-called converter circuits are used which, relative to a center potential, generate a positive and negative voltage in a DC voltage intermediate circuit having two capacitors or capacitances connected in series, wherein the voltages of the respective capacitors can be set or regulated independently of one another. Imbalances in the supply voltage system or generator can thus be compensated for. For this purpose, each capacitor has an assigned boost converter circuit.
A circuit arrangement 500 on which the invention is based is known, e.g., from EP 1 313 206 B1 and shown in FIG. 4 (without connecting lines 11a, b—FIG. 3). The three phases P1 to P3 of a generator 502 are connected to an inverter 506 via an inductor block 504 or respectively via an inductor L1 to L3. In this case, the three phases P1 to P3 having a peak voltage US are passed to a respective center connection 512a-c of three half-bridges 510a-c of the inverter 506. From each center connection, a Top switch 516a-c leads to a common Top connection 514 and a Bot switch 516a-c leads to a common Bot connection 515. In this case, each switch comprises an IGBT 520 with diode 522 reverse-connected in parallel.
An intermediate circuit 508 is connected between Top connection 514 and Bot connection 515, intermediate circuit 508 containing two series-connected capacitors 524a, b, the center connection 526 of which is connected to the center connection M of the generator 502.
By way of example, it is known from U.S. Pat. No. 6,567,283 B2 to insert between a diode rectifier and intermediate circuit 508 two series-connected Top and Bot IGBTs with freewheeling diodes, which are connected in parallel with the capacitors 524a, b via converter diodes lying in the Top and Bot connections 514, 515. The short-circuiting of the generator for boost purposes is thus transferred from the inverter into a separate boost branch.
In accordance with FIG. 5, a further circuit arrangement is known from U.S. Pat. No. 6,879,053. The phases P1 to P3 are in this case conducted to a diode rectifier 550 via an inductor block 504. The rectified voltage is then passed to a DC-DC converter 552, which charges capacitors 524a, b. The DC-DC converter 552 contains two step-up converters 554a, b, one each per series-connected capacitor 524a, b. 
The driving of the known booster converters is relatively simple to implement since they act on the already rectified voltage and no longer have to be synchronized with the generator phases. However, with three switch half-bridges on three diode half-bridges and four switch half-bridges, the number of semiconductor modules required is considerable.
This is because in practice fully equipped IGBT half-bridges with freewheeling diodes are in each case used for the series connection of a switch and the associated converter diode, wherein the IGBT connected in parallel with the converter diode remains unused.